Reading devices for transponders are known through the German patents DE 40 03 410 C2 and DE 41 34 922 C1. These reading devices are suitable for reading and programming without contact with the transponders of the kind described in the German patent DE 40 03 410 C2.
The field of application of these reading devices and associated transponders includes technology for identifying people, vehicles, possessions and other objects. The use of this technology has increased in recent years. Depending on the application, the transponders are attached to people, vehicles and possessions, etc., and detected by at least stationary readers.
Frequently persons carry the transponders on a key chain or in a trouser pocket or also in the breast pocket, when the transponder has the shape of a credit card.
The readers, which are allocated to the transponders, have an antenna which serves as the transmitting-receiving antenna, and an evaluating unit that is either separate from the antenna or integrated in the antenna. The evaluating unit serves, among other things, to evaluate the data received from the transponder.
The transponder does not have its own battery for energy supply. Rather it draws its energy from the energy transferred from the reader via the antenna to the transponder. The transmission of data from the transponder to the reader is also carried out via the antenna by means of absorption modulation.
Despite the fact that the transponder lacks its own energy supply, there is the desire with non-contact transponders to make the effective radius or distance between the reader and the transponder as large as possible. The larger the distance, the more convenient is a reading point. Thus a reader can be passed, for the purpose of identification without it being necessary, for example, to pull the briefcase or the key chain (in general that object that contains the transponder) out of one's clothes and to bring the object into the vicinity of the reader.
In everyday practice, electricians usually install the non-contact reading system. Frequently, when the reader is mounted stationarily, the antenna is installed on the building site at a predetermined reading point in the desired wall of the building concerned even before the final cleaning operation. Not until all of the other construction work has been completed, are the other components, viz. the other components of the reader and transponder, added and the entire system is put into service.
It has been demonstrated that there are always problems once the system is put into service. The antenna resonant circuit, which is associated with the antenna, comprises an inductance, designed as a coil, and a capacitance, formed by means of a capacitor. If such an antenna resonant circuit is attached, e.g., to or in a wall as a room antenna, the antenna resonant circuit is mistuned due to the capacitance of the wall. Thus, the antenna resonant circuit is no longer tuned to a preset resonance frequency.
Depending on the nature of the wall, the capacitance of the wall and thus also the mistuning of the antenna resonant circuit varies. This problem also applies to antennas embedded in a roadway or street. In this case, external influences such as moisture, ice, snow or also the general nature of the soil are responsible for the mistuning of the antenna of the reader.
This eventual mistuning of the antenna resonant circuit works against the goal of making the distance between transponder and reader as large as possible for identification. Namely if the antenna resonant circuit of the transponder and the antenna resonant circuit of the reader are not trimmed to maximum on the same resonance frequency, the coupling conditions between both antenna resonant circuits deteriorate. Even if the antenna of the reader is just slightly mistuned, the possible distance of the reader declines dramatically.
If, in contrast, the antenna resonant circuit of the reader is tuned in such a manner that it resonates on the same resonance frequency as the antenna resonant circuit of the transponder, the results are ideal coupling conditions owing to the overvoltage of resonance. These conditions allow a long read distance between the transponder and the reader.
Of course, it is already known to equip the reading device with a variable capacitor, in order to affect the antenna resonant circuit when the antenna becomes mistuned following installation and to adapt to the conditions of the location of installation, in order to thus again tune the antenna resonant circuit to the specified resonance frequency. However, the use of such variable capacitors has not been successful in everyday practice.
Rather experience has shown that the profession of installers or service technicians is overtaxed with the trimming work that has to be done. Rather this profession knows very little about the technical fields of antennas and high frequency technology, a state that results in this profession avoiding the after-tuning of the antenna resonant circuit with the aid of the variable capacitor. Not only is a screwdriver required for tuning, but also suitable meters such as field strength meters or an oscillograph with a special receiving antenna must be on hand; however, they are usually not available. In the final analysis, the necessary tuning of the installed antenna is not done. For these reasons there are hardly any non-contact systems with large frame antennas providing the desired long read distances of up to 1 m.
The invention remedies this drawback. The invention is directed towards the problem of providing a reading device of the aforementioned kind that enables long distances between the reading device and the transponder, using the reader and the transponder as prescribed.
This problem is solved by means of the present invention.